1. Field of the Invention
The present invention relates to spray formed articles made of boron steel, and methods for making the same.
2. Background Art
It is known to make spray formed articles such as a spray formed tool. A particularly common spray forming technique involves thermally spraying steel particles onto a spray forming pattern, allowing the steel particles to cool to form a desired article, and separating the article from the spray forming pattern. The steel particles being thermally sprayed onto the spray forming pattern are typically sprayed from carbon steel feedstock having a carbon content of 0.7-1.0% by weight, based on the weight of the feedstock.
While the use of carbon steel feedstock having a carbon content of 0.7-1.0% by weight typically results in metal articles which have relatively good surface and structural characteristics, the resulting articles can also have relatively high levels of oxides, typically on the order of about 25% by volume. The resulting articles can also have relatively high porosity. Tensile strength, weldability, and fracture toughness are believed to be inversely related to the level of oxides and porosity.
Most spray formed metal articles made in accordance with conventional techniques and materials typically have tensile strengths on the order of about 20-25 kpsi and an average Rockwell C micro-hardness of about 40. Metal articles with these properties are typically suitable for many prototype stamping and tool forming applications. However, there are commercially significant other applications that articles with these properties, because of the relatively low strength, are not typically suitable for use with. For instance, plastic injection molding tools are typically required to have tensile strengths above about 30 kpsi.
Accordingly, it is desirable, and there is a need, to provide spray formed articles and a method for making the same that provides an improvement over the prior art.
At least one aspect of the present invention relates to a method of spray forming a boron steel article. The method comprises providing a spray forming pattern, spraying metallic particles of boron steel onto the spray forming pattern, and cooling the metallic particles to form an article made of boron steel on the spray forming pattern. In this embodiment, the particles of boron steel are sprayed from a carbon steel feedstock comprising 0.25-2.25 weight percent boron, based on the total weight of the carbon steel feedstock.
In another aspect of the present invention, the metallic particles are sprayed from a carbon steel feedstock have a boron content of 0.25-1.25 weight percent, based on the total weight of the feedstock, and a titanium content of 0.50-2.0 weight percent, based on the total weight of the feed stock.
In yet another aspect of the present invention, the present invention pertains to a method of spray forming a metal article comprises providing a spray forming pattern, spraying metallic particles onto the spray forming pattern and allowing the sprayed metallic particles to cool to form a metal article having an average Rockwell C micro-hardness of greater than 45.
The present invention also relates to articles made of boron steel. The articles may be formed by the methods described herein.